The present invention generally relates to a focusing error detection apparatus, and more particularly to an apparatus for detecting a focusing error of a converging light beam on a recording medium in a light-beam scanning image forming system, which apparatus is applicable to an image forming system which may be a digital copier, a digital color copier or the like.
A laser-beam scanning image forming system forms an image on a recording medium which is scanned by a converging laser beam modulated in accordance with an image signal. The recording medium is, for example, a photosensitive drum on which an electrostatic latent image is formed by the exposure to the laser beam. In order to form a high quality image by the laser-beam scanning image forming system, it is necessary to locate a focusing position of a laser beam (which position is a so-called "beam waist" position) on a surface of the recording medium at a very high accuracy. However, when environmental temperatures are changed significantly, optical parts in the image forming system or mounting parts to which the optical parts are secured are more or less expanded or shrinked, and refraction indexes of the optical parts are varied from the preset values. Thus, even if relative positions between the parts in the image forming system are adjusted in order to locate the focusing position of the converging laser beam on the recording medium at a very high accuracy, the focusing position may deviate from an intended position on the surface of the recording medium due to the change of environmental temperatures. Accordingly, there is a problem in that a spot diameter of the laser beam on the recording medium surface is varied from an intended spot diameter, thereby the quality of the image formed by the image forming system becoming worse.
In order to solve the above mentioned problems in the prior art, Japanese Laid-Open Patent Application No. 1-237614, for example, discloses a converging position detecting device. The converging position detecting device includes first and second photodetectors being aligned along a scanning line at a given interval, two slits each having a slit opening that is smaller than the spot diameter of a converging light beam and the two slits placed in contact with the first and second photodetectors respectively, and a signal detection circuit for comparing waveforms of signals outputted by the photodetectors and detecting a deviation of the converging light beam from an intended focusing position on a recording medium being scanned. The first and second photodetectors each have a width of its light receiving area that is smaller than the spot diameter of the converging light beam, the first photodetector being placed at a height below the surface of the recording medium and the second photodetector being placed at the same height above the recording medium surface. Accordingly, this conventional focusing error detection apparatus detects a deviation of the converging light beam from the intended focusing position on the recording medium. However, this conventional focusing error detection apparatus requires the use of two small photodetectors and two slits in contact with the photodetectors, the two slits each having an opening smaller than the spot diameter of a converging light beam, and there is a problem in that a malfunction is likely to occur in the conventional apparatus owing to a clogging of foreing matter in the slits.
Also, Japanese Laid-Open Patent Application No. 2-93619 discloses a laser beam scanning device for detecting the spot size of a laser beam and detecting a deviation of the laser beam in a sub scanning direction perpendicular to the rotation axis of a photosensitive drum as the recording medium. The laser beam scanning device includes a first detecting part for detecting the spot size of the laser beam, and a second detecting part for detecting a deviation of the laser beam in the sub scanning direction. The first detecting part has a first photodetector and a first cover in contact with the photodetector, the first cover being formed with a specially shaped opening that allows a width of a light receiving area of the first photodetector to be gradually changed in the main scanning direction. The second detecting part has a second photodetector and a second cover in contact with the second photodetector, the second cover being formed with a specially shaped opening that allows a width of a light receiving area of the second photodetector to be gradually changed in the sub scanning direction. This conventional focusing error detection apparatus detects a change in the spot diameter of the laser beam from a waveform of a signal outputted by the first photodetector, thus detecting a deviation of the laser beam from an intended focusing position on the photosensitive drum. However, this conventional focusing error detection apparatus detects only a deviation of the laser beam in the sub scanning direction perpendicular to the rotation axis of the photosensitive drum, but it cannot detect a deviation of the laser beam in the main scanning direction from an intended focusing position on the photosensitive drum and a deviating direction thereof.
As described above, in a light-scanning image forming system in which a recording medium is exposed to and scanned by a converging light beam for recoding image data, it is important to detect accurately the focusing position of the converging light beam on the recording medium, in order to form an image with high quality. Especially, a high-density laser-beam scanning image forming system, in which a converging laser beam scans the recording medium and a spot of the laser beam with a very small diameter is cast on the surface of the recording medium, must locate the focusing position (or, the beam waist position) of the converging laser beam on the recording medium surface at extremely high accuracy.
However, even if optical parts in the image forming system are already adjusted for setting a focusing position of the converging laser beam at a very high accuracy, the setting positions of the optical parts, the refraction indexes of the optical parts or the wavelength of the laser beam emitted by the light source may be varied due to thermal expansion or shrinkage of the optical parts when environmental temperatures are changed, thereby the focusing position of the converging laser beam deviating from an intended position on the surface of the recording medium. Such a deviation of the focusing position makes a spot diameter of the converging laser beam on the surface of the recording medium to be varied from an intended spot diameter thereof, and the quality of the image becomes worse, for example, the resolution ability deteriorates or the dot density of the formed image is varied.
Therefore, after the relative positions between the optical parts in the image forming system are adjusted and the focusing position of the converging laser beam is preset, it is necessary to further correct the positions of the optical parts and the light source in the image forming system by fine adjustment in response to the deviation of the focusing position, so that the converging laser beam is always focused on the intended focusing position on the surface of the recording medium at a high accuracy. For this reason, it is necessary to detect accurately a focusing error of a converging light source on the surface of the recording medium in the image forming system.